Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 3,505.9	$ 3,238.3	$ 6,829.1	$ 6,315.5
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,450.4	2,297.8	4,788.6	4,477.2
PEO
Disaggregation of Revenue [Line Items]
Revenues	1,058.2	942.4	2,046.0	1,842.2
Other
Disaggregation of Revenue [Line Items]
Revenues	(2.7)	(1.9)	(5.5)	(3.9)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,599.4	1,512.7	3,114.7	2,937.0
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,599.6	1,512.5	3,116.4	2,937.8
HCM | PEO
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
HCM | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.2)	0.2	(1.7)	(0.8)
HRO, excluding PEO benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	614.2	542.8	1,176.7	1,051.9
HRO, excluding PEO benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	232.9	210.7	463.4	417.1
HRO, excluding PEO benefits pass-throughs | PEO
Disaggregation of Revenue [Line Items]
Revenues	383.8	334.2	717.1	637.9
HRO, excluding PEO benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(2.5)	(2.1)	(3.8)	(3.1)
PEO Benefits Pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	673.2	607.1	1,326.6	1,202.3
PEO Benefits Pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
PEO Benefits Pass-throughs | PEO
Disaggregation of Revenue [Line Items]
Revenues	673.2	607.1	1,326.6	1,202.3
PEO Benefits Pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Global
Disaggregation of Revenue [Line Items]
Revenues	490.0	469.0	963.5	918.2
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	490.0	469.0	963.5	918.2
Global | PEO
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Global | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Client Fund Interest
Disaggregation of Revenue [Line Items]
Revenues	129.1	106.7	247.6	206.1
Client Fund Interest | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	127.9	105.6	245.3	204.1
Client Fund Interest | PEO
Disaggregation of Revenue [Line Items]
Revenues	1.2	1.1	2.3	2.0
Client Fund Interest | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0